1. Field of the Invention
The present invention relates to a blood pressure monitoring apparatus and method for continuously monitoring a blood pressure of a living subject.
2. Related Art Statement
There is known, as a blood pressure measuring apparatus for non-invasively measuring an intraarterial blood pressure of a subject, a so-called Korotokoff-sound blood pressure measuring apparatus or oscillometric blood pressure measuring apparatus. The Korotokoff-sound blood pressure measuring apparatus determines a blood pressure of the subject, based on a pressing pressure of a pressing band (a cuff) being wound around a portion of the subject at the time of occurrence or disappearance of Korotokoff sounds produced by changing the pressing pressure of the pressing band. The oscillometric blood pressure measuring apparatus determines a blood pressure of the subject, based on variation of amplitude of a pulse wave produced while the pressing pressure of the pressing band is changed.
In an operating room, an intensive care unit, or the like, it is needed to successively measure a blood pressure of a subject when an urgent medical treatment or cure is required. In the case where the above conventional blood pressure measuring apparatus is used, it takes several tens of seconds from a start of the blood pressure measurement to obtain a blood pressure of the subject. Moreover, if an interval between successive blood pressure measurements is shortened to obtain a blood pressure at a relatively short period, congestion occurs to a body portion of the subject due to high frequency of pressing of the pressing band, whereby errors occur to the blood pressure measurements.
Further, there has been proposed a blood pressure monitoring apparatus including means for calculating, based on a signal non-invasively obtained from a subject, a time of propagation of a pulse wave which propagates through an artery of the subject, and means for starting a blood pressure measurement with a pressing band (cuff), when an amount of change of the propagation time does not fall within a predetermined alarm range. An example of the blood pressure monitoring apparatus is disclosed in Laid-open Publication No. 7-313472 of unexamined Japanese Patent Application. In the disclosed apparatus, the distress of the subject due to the pressing of the pressing band is decreased, and a blood pressure measured using the pressing band is automatically obtained when the amount of change of the propagation time does not fall within the predetermined alarm range.
However, the above blood pressure monitoring apparatus discloses only a technique to start a blood pressure measurement with the pressing band when an amount of change of pulse-wave propagation time which changes in relation with the blood pressure of the subject does not fall within the alarm range. In this case, it takes several tens of seconds from the start of the blood pressure measurement with the pressing band, to obtain a blood pressure of the subject. Thus, the above apparatus can not speedily obtain a blood pressure of the subject when the amount of change of pulse-wave propagation time indicates an abnormal change of the blood pressure of the subject. Therefore, it is difficult to use the above blood pressure monitoring apparatus in the operating room, the intensive care unit, or the like, in which an urgent medical treatment or cure is required.